The reaction between a soluble silica source and a soluble alumina source for making cation exchange systems and molecular sieves is known. However, the reaction product can vary widely depending upon the molar ratios of silica and alumina, and the reaction conditions.
U.S. Pat. No. 4,357,165 describes how to make self-setting, no-bake foundry binders, based upon a soluble silica source and a soluble alumina source, which cure under ambient conditions. These binders are amorphous hydrogels and remain amorphous hydrogels after curing because the water in the solution is not substantially removed during the curing process. The patent states at column 9, line 66 to column 10, line 4 that no heating or baking of the mold is required, and that, in fact, such treatment may significantly reduce subsequent moisture resistance, storage strength, and resistance to scabbing if introduced before the mold has had a chance to cure. Again at column 11, lines 15-16, the patent states that it is found that high temperatures during the cure period can cause a decrease in tensile strength.
These binders have the following deficiencies:
(1) low strength and long setting time; PA1 (2) excess of free alkalis which is disadvantageous for reclamation; PA1 (3) poor humidity resistance; PA1 (4) high binder level (up to 6 to 7 weight percent based upon the weight of the sand) is required for effective use; and PA1 (5) calcined kaolin must be added to improve the hot strength. PA1 (1) a soluble source of silica, and PA1 (2) a soluble source of alumina, such that the source of silica, source of alumina, or both contain an alkali metal.
Because of these deficiencies, these binder systems are not only impractical for self-setting, no-bake application, but are also useless for high production core and mold making.